Active beam scanning antennas are now being deployed in cellular communications systems. An active beam scanning antenna may be located at or near a cellular base station, and may adjust the amplitude and/or phase of the sub-components of a signal that is to be transmitted through the antenna. By adjusting the amplitudes and/or phases of the signal sub-components, the shape and pointing direction of the antenna beam formed by the active beam scanning antenna may be changed. Thus, the antenna may, for example, create different antenna beams during different time slots that point in different directions to provide high directivity antenna beams on a time slot-by-time slot basis that point at individual mobile users or groups of mobile users.
As shown in FIG. 1, an active beam scanning antenna 10 may include an antenna interface unit 11, a calibration circuit board 12, phase shifters 13 and radiating units 14. Each radiating unit 14 may comprise, for example, a vertically-disposed column of individual radiating elements. Typically, the active beam scanning antenna 10 will include multiple phase shifters 13 and radiating units 14. The antenna interface unit 11 may connect the antenna 10 to a main control system. Signals that are to be transmitted by the active beam scanning antenna 10, and signals that are received by the active beam scanning antenna 10, may be passed through the antenna interface unit 11 to the main control system. The phase shifters 13 may be used to divide a radio frequency (“RF”) signal that is to be transmitted by the antenna into a plurality of sub-components and to adjust the phase differences between these sub-components of the RF signal. The sub-components are then fed to individual radiating elements (or sub-arrays of radiating elements) that are included in the respective radiating units 14. The phase differences may be adjusted in order to electronically adjust an elevation or downtilt angle of the active beam scanning antenna 10. In some cases, the phase shifters 13 may be omitted and replaced with a power divider circuit board that divides the RF signal to be transmitted into sub-components that are fed to the individual radiating elements (or sub-arrays of radiating elements) that are included in the respective radiating units 14. The power divider circuit board may also set fixed phase differences between the signals supplied to the individual radiating elements (or sub-arrays of radiating elements).
The calibration circuit board 12 may be interposed between the antenna interface unit 11 and the radiating units 14. Calibration test signals may be transmitted from the antenna interface unit 11 to the radiating units 14 via the phase shifters 13. The calibration circuit board 12 may extract portions of the respective calibration test signals that are transmitted to the radiating units 14. These extracted portions of the calibration test signals may be compared to a reference calibration test signal by the main control system to obtain the amplitude and phase of the sub-components of the RF signal that are fed to each of the radiating units 14. Weighted values for the amplitudes and phases of the sub-components of the signal that is to be transmitted may then be calculated that will provide a desired beam pattern.
When an antenna only includes radiating units that operate in one frequency band, inter-band isolation issues may not arise. However, when radiating units that operate at different frequency bands are integrated into the same antenna, interference between signals in the respective frequency bands may arise.